UK Border Agency (UKBA) staff dealt with a backlog of immigration cases so inefficiently that at one point 100,000 pieces of post were unopened, a report has said.

Chief Inspector of Borders and Immigration John Vine found that security checks were not properly carried out on old cases and that workers had failed to check the records of other Government departments.

Applications were placed into an archive of unresolved cases after "very minimal work", despite the agency assuring MPs that "exhaustive" checks had been carried out, he concluded.

Mr Vine said: "An examination of controlled archive cases showed that the security checks - which the agency stated were being done on these cases - had not been undertaken routinely or consistently since April 2011.

"I also found that no thorough comparison of data from controlled archive cases was undertaken with other Government departments or financial institutions in order to trace applicants until April 2012. This was unacceptable and at odds with the assurances given to the Home Affairs Select Committee that 124,000 cases were only archived after 'exhaustive checks' to trace the applicant had been made."

He was asked to evaluate how well UKBA had handled the backlog of thousands of unresolved immigration cases. In March 2011, there were 147,000 unfinished cases that were passed to an audit unit tasked with dealing with the backlog.

Mr Vine said he believed little had been done to try to resolve the cases before they were passed over. He said: "I found that updates given by the agency to Parliament in the summer of 2011, stating that the legacy of unresolved asylum cases was resolved, were inaccurate."

A Home Office spokesman said: "The backlog of asylum claims that was allowed to build up before 2006 was unacceptable. Since then, UKBA has worked its way through the backlog. Cases where, after checks, the agency was unable to trace the individual were placed in the controlled archive. Before they were put into the controlled archive, all cases were checked against the warnings index and the police national computer.

"UKBA's policy was that all cases in the controlled archive should be checked regularly against the warnings index and the police national computer. This report shows that these checks were not carried out.

"The UK Border Agency has now reviewed the cases within the controlled archive and undertaken a robust process to trace those we can find evidence are still in the UK and, where appropriate, remove them from the country."